Seeking ideal electron donor compounds has been being a hot point of the studies on novel polypropylene catalysts. Chinese patent application CN1313869A discloses the use of 2,3-dihydrocarbylsuccinate compounds as a catalyst component in the preparation of a solid catalyst for olefin polymerization, among these compounds 2,3-diisopropylsuccinates being especially preferred internal electron donor compounds. As internal electron donor of Ziegler-Natta catalysts, this class of succinates improves the ability of the catalysts controlling molecular weight distributions, isotactivities and oligomer content of olefin polymers. Current Petrochemical Industry, 2003, Vol. 11(10), 4-11 reports that an impact copolymer, which is produced by using a propylene polymerization catalyst containing such a 2,3-dihydrocarbylsuccinate as electron donor, has balanced rigidity and impact property.
CN1313869A further discloses a process for the synthesis of the 2,3-dihydrocarbylsuccinates, comprising esterification, alkylation, reduction, oxidative coupling, SN2 coupling, and combinations thereof. For example, CN1313869A discloses in Example 18 the preparation of diethyl 2,3-dipropylsuccinate through the oxidative coupling of ethyl pentanoate under the action of diisopropylamido lithium and TiCl4, and in Example 22 the preparation of diethyl 2,3-dicyclohexylsuccinate through the esterification of 2,3-diphenylsuccinic acid and the reduction of phenyl to cyclohexyl.
However, for the preparation of 2,3-non-linear-alkyl-succinate compounds, no ideal process is disclosed in the prior art. The non-linear-alkylation, especially isoalkylation or sec-alkylation, of a diester of succinic acid on 2- and 3-positions are generally difficult due to large steric hindrance, and the yield is relatively low. The oxidative coupling method mentioned in CN1313869A requires the use of diisopropylamido lithium (LDA) prepared from hazardous material, n-butyl lithium, and the reaction is carried out at a temperature of −70° C.
As for the synthesis of a 2,3-di(non-linear-alkyl)-2-cyano-succinate compound, although literatures, J. Am. Chem. Soc. 1952, 74, 1056-1059 and Bull. Soc. Chim. Fr. 1975, (9-10, Pt. 2), 2189-2194, report referable methods, for example, one wherein a 2,3-dialkyl-2-cyanosuccinate compound may be prepared by alkylating a 3-alkyl-2-cyanosuccinate compound with a bromoalkane in three times amount or an expensive iodoalkane as alkylating agent, the product 2,3-dialkyl-2-cyanosuccinate compound has a boiling point close to the boiling point of the reactant 3-alkyl-2-cyanosuccinate compound so that isolation process such as rectification are difficult to be carried out, and for a reactant having large steric hindrance, such as 3-non-linear-alkyl-2-cyanosuccinate compound, the non-linear-alkylation on 2-position is more difficult.
Thus, there is still need for a novel process for the synthesis of a 2,3-di(non-linear-alkyl)succinate compound, which process may provide the 2,3-di(non-linear-alkyl)succinate compound under mild conditions at a relatively high yield. Furthermore, there is also need for a process that can prepare at industrial scale 2,3-di(non-linear-alkyl)-2-cyano-succinate compound as an intermediate of the 2,3-di(non-linear-alkyl)succinate compounds.